Conventional magnetic disk drive designs comprise a commonly denominated Contact Start-Stop (CSS) system commencing when the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates, such that the head can be freely moved in both the circumferential and radial directions allowing data to be recorded on and retrieved from the surface of the disk at a desired position. Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from the stop and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic operation consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk and stopping.
It is considered desirable during reading and recording operations to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. Thus, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head, thereby permitting the head and the disk to be positioned in close proximity with an attendant increase in predictability and consistent behavior of the air bearing supporting the head. However, if the head surface and the recording surface are too flat, the precision match of these surfaces gives rise to excessive restriction and friction during the start up and stopping phases, thereby causing wear to the head and recording surfaces eventually leading to what is referred to as a "head crash." Thus, there are competing goals of reduced head/disk friction and minimum transducer flying height.
Conventional practices for addressing these apparent competing objectives involve providing a magnetic disk with a roughened surface to reduce the head/disk friction by techniques generally referred to as "texturing."Conventional texturing techniques involve polishing the surface of a disk substrate to provide a texture thereon prior to subsequent deposition of layers, such as an underlayer, a magnetic layer, a protective overcoat, and a lubricant topcoat, wherein the textured surface on the substrate is intended to be substantially replicated in the subsequently deposited layers.
The escalating requirements for high areal recording density impose increasingly greater requirements on thin film magnetic media in terms of coercivity, restriction, squareness, low medium noise and narrow track recording performance. In addition, increasingly high density and large-capacity magnetic disks require increasingly smaller flying heights, i.e., the distance by which the head floats above the surface of the disk in the CSS drive. The requirement to further reduce the flying height of the head challenges the limitations of conventional technology for controlled texturing to avoid head crash.
Conventional techniques for providing a disk substrate with a textured surface comprise a mechanical operation, such as polishing. In texturing a substrate for a magnetic recording medium, conventional practices comprise mechanically polishing the surface to provide a data zone having a substantially smooth surface and a landing zone characterized by topographical features, such as protrusions and depressions. See, for example, Nakamura et al., U.S. Pat. No. 5,202,810. Conventional mechanical texturing techniques, however, are attendant with numerous disadvantages. For example, it is extremely difficult to provide a clean textured surface due to debris formed by mechanical abrasions. Moreover, the surface inevitably becomes scratched during mechanical operations, which contributes to poor glide characteristics and higher defects. Such relatively crude mechanical polishing, with attendant non-uniformities and debris, does not provide a surface with an adequately specular finish or with adequate microtexturing to induce proper crystallographic orientation of a subsequently deposited magnetic layer on which to record and read information, i.e., a data zone.
Data zones are conventionally provided with a smooth specular finish or with a mechanically textured surface. In mechanically texturing a substrate surface for data recordation and reading, i.e., a data zone, deep scratches are formed for inducing a desired magnetic orientation. However, mechanical texturing disadvantageously results in non-uniform scratches believed to be due to non-uniform particle sizes of abrasive material ranging from about 0.1 .mu.m to about 5 .mu.m. In addition, various desirable substrates are difficult to process by mechanical texturing. This undesirably limiting facet of mechanical texturing, virtually excludes the use of many materials for use as substrates.
Laser technology has been employed to texture a substrate surface to form a topography suitable for a landing zone. Such landing zone laser technology typically comprises impinging a pulsed, focused laser light beam on a non-magnetic substrate surface. Laser textured landing zones typically exhibit a topographical profile comprising a plurality of spaced apart protrusions extending above the substrate surface or a plurality of spaced apart depressions extending into the substrate surface. See, for example, Ranjan et al., U.S. Pat. No. 5,062,021, wherein the disclosed method comprises polishing an NiP plated Al substrate to a specular finish, and then rotating the disk while directing pulsed laser energy over a limited portion of the radius, to provide a textured landing zone leaving a specular data zone. The landing zone comprises a plurality of individual laser spots characterized by a central depression surrounded by a substantially circular raised rim.
Another laser texturing technique is reported by Baumgart et al. "A New Laser Texturing Technique for High Performance Magnetic Disk Drives," IEEE Transactions on Magnetics, Vol. 31, No. 6, pp. 2946-2951, November 1995. See, also, U.S. Pat. Nos. 5,550,696 and 5,595,791
In copending application Ser. No. 08/666,374 filed on Jun. 27, 1996 a laser texturing technique is disclosed employing a multiple lens focusing system for improved control of the resulting topographical texture. In copending application Ser. No. 08/647,407 filed on May 9, 1996, a laser texturing technique is disclosed wherein a pulsed, focused laser light beam is passed through a crystal material to control the spacing between resulting protrusions.
In copending PCT application Ser. No. PCT/US96/06830, a method is disclosed for laser texturing a glass or glass-ceramic substrate employing a laser light beam derived from a CO.sub.2 laser source. The textured glass or glass-ceramic substrate surface comprises a plurality of protrusions which extend above the substrate surface, without surrounding valleys extending substantially into the substrate as is characteristic of a laser textured metallic substrate. The effect of laser parameters, such as pulse width, spot size and pulse energy, and substrate composition on the protrusion or bump height of a laser textured glass or glass-ceramic substrate is reported by Kuo et al., in an article entitle "Laser Zone Texturing on Glass and Glass-Ceramic Substrates," presented at The Magnetic Recording Conference (TMRC), Santa Clara, Calif., Aug. 19-21, 1996.
In copending application Ser. No. 08/796,830 filed on Feb. 7, 1997, a method is disclosed for laser texturing a glass or glass-ceramic substrate, wherein the height of the protrusions is controlled by controlling the quench rate during resolidification of the laser formed protrusions. One of the disclosed techniques for controlling the quench rate comprises preheating a substrate, as by exposure to a first laser light beam, and then exposing the heated substrate to a focused laser light beam.
As areal recording density increases the flying height must be reduced accordingly, thereby challenging the limitations of conventional laser texturing technology for uniformity and precision in forming a textured landing zone comprising a plurality of protrusions. The requirements for continuous alignment and adjustment of a laser beam are exacerbated in geographic locations with relatively unstable environmental conditions, such as temperature, vibration and shock, particularly in regions susceptible to seismological disturbances such as tremors and earthquakes. Conventional laser delivery systems for texturing a landing zone comprise a system of mirrors and lenses which must be precisely and accurately maintained, particularly as the flying height is reduced to a level of less than about 300 .ANG., due to inherent undulations of the substrate surface. Uniform and precise texturing require continuous maintenance of alignment of a system of mirrors and lenses. It is extremely difficult to maintain the requisite precise alignment and satisfy the reduced flying height requirements for high areal recording density, particularly in geographical locations subjected to environmental changes, and seismological disturbances.
In copending application Ser. No. 08/954,585, filed on Oct. 20, 1997 an apparatus and method are disclosed for laser texturing a substrate employing a fiber-optic laser delivery system wherein sub-laser beams are passed through plural fiber optic cables and microfocusing lens to impinge on opposite surfaces of a rotating substrate. The use of a fiber optic cable delivery system facilitates alignment and reduces maintenance, even in geographical areas subject to environmental changes, particularly seismological disturbances. In copending application Ser. No. 08/955,448 filed on Oct. 21, 1997 an apparatus and methodology is disclosed wherein inherent variations in the surface topography of a disk substrate, such as variations in surface planarity, e.g., surface runout, are detected and a laser parameter adjusted in response to the detected surface variation. Such controlled texturing parameters include laser power, pulse duration, repetition rate and/or the distance between the microfocusing lens and the substrate surface.
Thus, conventional practices in texturing a substrate, e.g., a non-magnetic substrate or underlayer provided thereon, comprise decoupling the magnetic requirements (data zone on which information is recorded and read) from the mechanical requirements (landing zone), by forming a dedicated landing zone where the slider is parked and latched after the drive has been shut down. Baumgart et al. "Safe landings: Laser texturing of high-density magnetic disks" Data Storage, March 1996; U.S. Pat. No. 5,550,696 issued to Nguyen; and U.S. Pat. No. 5,595,791 issued to Baumgart et al. Accordingly, laser texturing has been employed to provide a landing zone having a topographical profile comprising a plurality of spaced apart protrusions extending above the substrate surface or a plurality of spaced apart depressions extending into the substrate surface. Typically, such laser texturing to provide a landing zone is performed on a non-magnetic substrate which has previously been polished by the substrate manufacturer to provide a specular or smooth surface which serves as the data zone.
Lasers have also been employed to inscribe a plurality of indelible grooves in a surface of a magnetic recording medium to function as optical servo tracks. Williams et al. U.S. Pat. No. 5,120,927. Laser techniques have also been employed to remove particular contamination from surfaces. Tam, "Laser-cleaning techniques for removal of surface particulates", J. Appl. Phys. 71 (7), Apr. 1, 1992, pp. 3515-3523.
It is recognized that the most significant magnetic properties of thin-film media are the remanence-thickness product (M.sub.r t), coercivity (H.sub.c) and coercive squareness (S*). The concept of orientation ratio (OR) has been defined for magnetic media as a means to quantify and understand the directional nature of magnetic properties of a magnetic recording medium. Thus, the most common definition of OR is the ratio of M.sub.r t, H.sub.c or S* in the tangential direction to values in the radial direction. Thus, in-plane anisotropies impact the OR for a particular magnetic recording medium including scratch anisotropies. Thus, the mechanism for the OR results from a geometric effect and is based upon the preferential growth of crystallite chains due to a self-shadowing mechanism stemming from mechanical polishing. Johnson et al., "In-Plane Anisotropy in Thin-Film Media: Physical Origins of Orientation Ratio (Invited)", IEEE Transactions on Magnetics, Vol. 31, No. 6, Nov. 1995, pp. 2721-2727.
Circumferential polishing or texturing of rigid disk substrates provides anisotropy in thin film magnetic disks which enhances coercivity in the track direction. The orientation in textured or polished thin film disks can be generated by a magnetostatic effects arising from effective decoupling in the cross track direction. Such magnetostatic anisotropy increases in strength as the polishing grooves become finer and deeper. This effect is stronger where there is chain growth along the track direction. Miles et al. "Micromagnetic Simulation of Texture Induced Orientation in Thin Film Media", IEEE Transactions on Magnetics, Vol. 31, No. 6, Nov. 1995, pp. 2770-2772.
As the requirements for high areal recording density increase, the need for improved substrate texturing for inducing magnetic orientation in a data zone increases. Such improved texturing requires fine, deep and uniform scratches which cannot be achieved employing conventional mechanical polishing techniques. Accordingly, there exists a need for improved methodology for texturing a non-magnetic substrate or underlayer thereon of a magnetic recording medium to provide a data zone with precisely formed uniform topographies to induce the requisite magnetic orientation in a subsequently applied magnetic layer.